Authorities reconstruct bombing suspects’ actions after their photos are made public

As their photographs spread rapidly across the Internet, the Tsarnaev brothers decided to make their move. Not waiting for police to find them, they gathered guns and homemade explosives for what became a final, bloody rampage on a community still in shock from the bombings three days before.

In less than 15 minutes late Thursday, authorities said, the brothers fatally shot a campus police officer as he sat in his car, then carjacked a Mercedes-Benz sport utility vehicle at gunpoint. They held the driver hostage for 30 minutes as they scoured Boston’s western suburbs for bank machines from which to take the man’s cash.

Finally discovered, the Tsarnaevs led police on a chase through residential streets, hurling pipe bombs as they drove. When cornered, they battled police with guns and more homemade explosives, wounding a transit officer and trading more than 200 rounds until the officers ran out of ammunition.

The drama ended for the elder brother, Tamerlan, 26, when police shot him and then he apparently was run over by his younger brother, Dzhokhar, in a melee witnessed by scores of residents of Watertown, Mass., a residential community less than eight miles from the apartment where the pair lived.

Dzhokhar Tsarnaev, 19, fled on foot, somehow eluding police and beginning a widespread search by thousands of federal agents and state and local officers. The hunt shut down much of metropolitan Boston for hours and transfixed millions of Americans who watched the events unfold.

On Friday night, soon after state police held a news conference saying they did not know where Dzhokhar Tsarnaev was hiding, he was discovered in a plastic-covered boat parked in the yard of a Watertown home. Police surrounded the boat and, after nearly three hours, brought him out alive.

The younger Tsarnaev, a former high school wrestler, was identified along with his brother as a suspect in Monday’s bombings near the crowded finish line of the Boston Marathon that killed three people and wounded more than 170.

It is unclear whether investigators knew the suspects’ names when they released photographs and videos of them along the marathon course around 5 p.m. Thursday in what the FBI said was an appeal for help from the public. Within minutes, the images circulated worldwide on the Internet and in continuous loops on television networks.

Law enforcement officials had hoped that a wide distribution of photos would bring clues. Instead, it appears to have jarred the Tsarnaevs into action. After apparently spending three days watching the aftermath of the bombings from nearby Cambridge, the two left their apartment within hours of the FBI news conference, heavily armed and prepared for a fight. Whether they intended to flee the area or provoke a confrontation isn’t clear.

The following account was provided by multiple law enforcement officials involved in the manhunt or the bombing investigation. Several of them spoke on the condition of anonymity because the probe is ongoing.

Just after 10:30 p.m. Thursday, the pair walked up to a parked police car at the Massachusetts Institute of Technology, where Sean Collier, a 26-year-old campus officer, was nearing the end of his 3-to-11 p.m. shift.